Recently, a communication system adopting Code Division Multiple Access (CDMA) as a multiple access method is operated in the field of a mobile communication system. Improvements are given by High Speed Downlink Packet Access (HSDPA) for speeding up a communication (downlink communication) from a radio base station to a mobile station. Such a mobile communication system adopts a retransmission control method called Hybrid Automatic Repeat reQuest (HARQ). In the HARQ, when detecting an error in data received from the radio base station, the mobile station performs a retransmission request to the radio base station, while holding the data that includes the error. When receiving the retransmitted data from the radio base station, the mobile station performs decoding processing by combining the held data that was previously received and the retransmitted data. This makes it possible to improve the benefit of the decoding processing. Thus, the number of retransmissions may be reduced.
In such a system the mobile station returns an ACKnowledgement (ACK) to the radio base station when no error is detected by error detection processing. The mobile station returns a Negative ACKnowledgement (NACK) when an error is detected. If the data detection ever fails (for example, if control information necessary for data decoding is not acquirable), the mobile station returns a Discontinuous Transmission (DTX) indicating that no data has been received (or detected). The radio base station may change the content of the retransmission depending on whether the response from the mobile station is a NACK or a DTX. For example, if the response is a DTX, the previously transmitted data is retransmitted as it is. If the response is a NACK, it is considered that only a necessary part of the previous transmission data is retransmitted. As a result, it is possible to further improve efficiency of retransmitting processing.
As signal expressions of ACK/NACK/DTX, it is considered that the ACK corresponds to a signal “1”, the NACK corresponds to a signal “0”, and the DTX corresponds to a signal “NULL” (signal of level 0). However, by utilizing only these signals for ACK/NACK/DTX, it is difficult to distinguish between ACK/NACK and DTX in the radio base station when the mobile station is strongly affected by noises. Therefore, when the mobile station transmits an ACK/NACK to the radio base station, there is a method for transmitting a given PRE signal and a POST signal together before and after the ACK/NACK. The radio base station may accurately distinguish between the ACK/NACK and the DTX by comparing the levels of the PRE signal and the POST signal and the level of the ACK/NACK signal which is between those two signals (for example, see 3rd Generation Partnership Project, “Physical layer procedures (FDD)”, 3GPP TS25.214, 2007-05, V7.5.0.)
Meanwhile, if the mobile station does not acquire control information and fails to detect the data, a method is considered in which re-acquisition of the control information is attempted, instead of requesting retransmission, by transmitting the DTX immediately to the radio base station. This is because that, when the control information is acquired from a common control channel of the downlink, it is considered that demodulation and decoding of the received signal may not be performed successfully because of failure of channel estimation.
That is, the channel estimation of the common control channel at a time of a certain time slot #n is performed based on the received signal to the time slot #n−1 at first. Thus, the accuracy of the channel estimation may not be very good. Meanwhile, if channel estimation processing is delayed by one time slot, it is possible to estimate the common control channel at the time of the time slot #n based on the received signal to the time slot #n. Thus, the estimation accuracy is expected to increase. Accordingly, the control information may be acquired successfully by performing re-demodulation and re-decoding of the signal of the common control channel. By this method, it is possible to reduce the number of times of retransmission from the radio base station to the mobile station (see, for example, Japanese Laid-Open Patent Publication 2007-180925).
The data acquisition from the received signal fails and then re-acquisition is attempted. As a result, if the data acquisition is successful, response information (ACK or NACK) to the acquired data is transmitted. At this time, it is preferable to add the given signals (a PRE signal and a POST signal) before and after the response information.
However, as a method for adding the given signals after receiving success of the data re-acquisition, there is a problem that signal (PRE information) transmission before the response information is too late because of processing delay accompanied with the data re-acquisition. This is because the usually permissible time from reception of the data to transmission of the response information is determined according to communication standards. If the signal (PRE signal) before the response information is missing because the transmission before the response information is too late, recognition accuracy of the response information at the receiving side is lowered, and then the efficiency of retransmission control is inhibited.